Audio/Visual (AV) systems are widely used for making presentations and displaying information in commercial settings, and are also increasingly used in the home for television applications. AV systems often may include a projection system that operates according to a sequential color mode, that is, a mode in which monochrome frames of primary color information are displayed rapidly in synchronized succession. Sequential color mode projection systems may include a rotating color wheel interposed between a light source and a Digital Micromirror Device (DMD). By varying the amount of time that light received through the color wheel is reflected to a display screen picture element (“pixel”) from each micromirror, the projection system is able to project sequences of full color picture frames onto the display screen.
Many sequential color mode projection systems include one or more fast-responding light valves together with some sort of color selecting means (for example, a color wheel) operating in a sequential color mode. In such systems, the light valve rapidly displays monochrome frames of primary color information (generally three times faster than the normal refresh rate used for non-sequential systems. For instance, to display a frame of a color image, the frame is displayed by projecting three rapid sub-frames, each sub-frame being of a single primary color. That is, to project one frame of a color image, a completely red image sub-frame, a completely green image sub-frame, and a completely blue image sub-frame are projected one after the other, very rapidly.
The rapid display of the monochrome sub-frame information on the light valve is synchronized with the colors passing through the color selecting means. This rapid succession of primary color images creates the impression of a single full color image in the mind of an observer due to the phenomenon of color persistence (that is, the well-known fact that the human eye averages together information received over periods of one-fifteenth of a second or thereabouts).
However, if the observer's eye is not stationary relative to the image during the display on such a device (due to natural rapid saccadic motion of the eye, for example), the relative motion will cause the successive primary color images to fail to overlap perfectly on the viewer's retina. In this case, the viewer will perceive incorrectly colored edges to bright objects, which will disappear when the relative motion ceases. This creates the impression of unpredictable brief flashes of color in situations where none was expected, a situation frequently referred to as “sequential color visual artifacts” or “rainbowing.” The phenomenon is worst for two or more small white objects spaced apart horizontally and displayed against a black background. The natural saccades of a viewer's eyes, which are most rapid in the horizontal direction, may be sufficient to cause strong and nearly continuous artifacts under such circumstances.
Efforts to minimize sequential color artifacts have included speeding up the sub-frame display frequency as much as by a factor of 10; however, such speeds are difficult to achieve in commercial systems due to processing bandwidth limitations of projection system components. For example, DMD devices typically have a response time of around 10 microseconds. Furthermore, DMD devices require processing bandwidth for functions other than sequential color artifact suppression such as, for example, image grayscaling. Therefore, existing projection systems have limited processing bandwidth available for suppression of sequential color artifacts without impacting other operational design goals. Many projection systems run at up to three times the required sub-frame display rate, and some systems run at five times the rate. However, even at these increased frequencies, color artifacts are still visible to the human eye. To eliminate sequential color visual artifacts may require a sub-frame display frequency of twenty times the normal rate, which is difficult to achieve in a commercially practical system.
Thus, there is a need for a method and an apparatus to reduce or eliminate sequential color artifacts in a projection system.